I did this by accident myself once, wrote a transient file larger than 2 GB. I used the utility star convert under the tools>convert>star convert drop down menu. Under Post file type select transient and that allows you to specify the number of time steps for the new file. Under output file type select binary again and give it a new name with the pstt extension. Then hit apply. Just a typical warning, make sure you make a backup of the file before you start messing with it. If you end up needing to run it again you can run for fewer time steps (a size you can open) and then restart off that file for the next set of time steps.